1. Field of the Invention
This invention relates to a process for stably and continuously producing a solid-liquid metal mixture (hereinafter referred to as a semi-solidified metal composition) having an excellent workability.
2. Description of the Related Art
As a means for continuously producing the semi-solidified metal composition, there is a well-known mechanical agitating process wherein molten metal is charged at a certain temperature into a space between inner surface of a cylindrical cooling agitation vessel and an agitator rotating at a high speed and vigorously agitated while cooling and then the resulting semi-solidified metal composition is continuously discharged from the bottom of the vessel (hereinafter referred to as an agitator rotating process) as disclosed, for example, in JP-B-56-20944 (relating to an apparatus for continuously forming alloys inclusive of non-dendritic primary solid particles). Furthermore, there is also a well-known process of using an electromagnetic force for the agitation of molten metal (hereinafter referred to as an electrormagnetic agitating process).
As disclosed in JP-A-4-238645 (relating to a process and apparatus for producing a semi-solidified metal composition, there is another process wherein molten metal is charged into a space between a rotating agitator composed of a cylindrical drum having a horizontally rotating axis and a cooling ability and a fixed wall member having a concave face along the outer periphery of the agitator and a discharging force is generated by shear strain at a solid-liquid interface produced through the rotation of the rotating agitator while cooling to continuously discharge the semi-solidified metal composition from a clearance at the bottom (hereinafter referred to as a single roll process).
In all of the above processes, the solid phase in the semi-solidified metal composition is formed by vigorously agitating molten metal (generally molten alloy) while cooling to convert dendrites produced in the remaining liquid matrix into a spheroidal shape such that dendritic branches are substantially eliminated or reduced.
As a working process for the thus obtained semi-solidified metal composition, there are known a thixocasting process wherein the semi-solidified metal composition is cooled and solidified and then reheated to a semi-molten state, a rheocasting process wherein the semi-solidified metal composition is supplied to a casting machine as it is, and so on.
If it is intended to work the semi-solidified metal composition by the thixo or rheo process, the castability is dependent upon the fraction solid during casting, size, shape and uniformity of primary crystal grains in the semi-solidified metal composition and the like. When the fraction solid during casting is too low (heat content is large), the mitigation of heat load as a great merit in the working of the semi-solidified metal composition is damaged, while when the fraction solid is too high, there are caused some problems such as an increase of working pressure required during casting, deterioration of filling property and the like. On the other hand, the castability is improved as the primary solid particles have a smaller particle size and a spheroidal shape and the dispersion of the primary solid particles becomes more uniform. Therefore, in order to manufacture sound worked products by improving the castability of the semi-solidified metal composition, it becomes important to control not only the fraction solid in the castability but also the particle size, shape and uniformity of the primary solid particles.
When the cooling rate is made higher to make the particle size of the primary solid particles fine in all of the above processes, the growth of a solidification shell becomes large and hence it is apt to cause problems such as a decrease of the cooling rate, coarsening of primary solid particles, deterioration of quality, stop of operation and the like.
In order to realize the production of the semi-solidified metal composition as an industrial process, it is important to stabilize the operation and to provide a good quality.
As a countermeasure for solving the above problems, JP-B-3-66958 (relating to a process for producing metal composition of slurry structure) proposes an agitator rotating process wherein a ratio of shear strain rate to solidification rate is held within a range of 2.times.10.sup.3 -8.times.10.sup.3. In this process, however, it is difficult to conduct continuous operations because the torque of the agitator is raised by contacting the solidification shell growing on the cooling wall surface of the agitation cooling vessel with the agitator, and also the semi-solidified metal composition having a given quality can not be obtained due to the change of the cooling rate accompanied with the growth of the solidification shell.
In the above single roll process described in JP-A-4-238645, sufficient cooling and shear strain effect can be provided by properly selecting the diameter and revolution number of the rotating agitator, and also the continuous discharge of the semi-solidified metal composition having a high viscosity and fraction solid can be facilitated. However, when using the rotating agitator having a large cooling rate, the solidification shell growing on the outer peripheral surface of the agitator becomes thicker and is scraped off by a scraping member in the form of a flake. Furthermore, the amount of the solidification shell scraped increases and is included into the semi-solidified metal composition, so that the quality and castability of the semi-solidified metal composition are considerably degraded.